1. Field of the Invention
The present invention relates to a process for electrolytically coloring previously anodized aluminum or aluminum base alloy (referred to simply as aluminum hereinafter) by subjecting the aluminum to a direct current electrolysis, with the aluminum as a cathode, in an electrolytic coloring bath containing a metallic salt.
2. Description of the Prior Art
One known process for electrolytically coloring aluminum is an anodic oxidation of aluminum in an aqueous solution containing an organic acid as disclosed in, for example, U.S. Pat. Nos. 3,031,387 and 3,486,991. Another process is an inorganic coloring process which comprises electrolyzing previously anodized alumimum in an electrolytic coloring bath containing a metallic salt. The latter process can be classified as an alternating current electrolysis (as disclosed in U.S. Pat. No. 3,382,160) and a direct current electrolysis (as disclosed in U.S. Pat. No. 3,761,362), depending upon the kind of electric current which is passed through the electrolytic coloring bath.
The process disclosed in U.S. Pat. No. 3,761,362 can be characterized as the coloration of previously anodized aluminum by subjecting the aluminum to a direct current electrolysis, with the aluminum as a cathode, in an electrolytic coloring bath containing a metallic salt.
In this direct current electrolysis, aqueous solutions containing a water-soluble metallic salt are used as the electrolytic coloring bath. The colors formed on the surface of the aluminum are bronze when nickel salts are used, reddish brown when copper salts are used, bronze to black when tin salts are used, bronze when cobalt salts are used and yellow when iron salts are used. However, in practicing this process on an industrial scale, contamination of the electrolytic coloring bath with impurities or variation of the pH of the bath gives rise to an unstable coloring process, and in some cases a spalling of the film results. This sometimes renders it difficult to obtain a consistently uniform, colored oxide film. Additionally, the metal element is sometimes deposited on the anodic oxide film disturbing the normal progress of the coloring.
Investigations were made on these phenomena and the following facts were discovered. When the coloring process is carried out using an electrolytic coloring bath containing, for example, water-soluble nickel salts, the colored oxide films obtained becomes paler in color as the concentration of sodium ion, which is introduced into the bath as an impurity, increases. The colored oxide film is finally spalled and thus further treatment becomes impossible. When tin salts are used, metallic tin is easily deposited on the oxide film. This phenomenon is particularly marked when the oxide film is uneven and is defective, with the result that a uniform, colored oxide film is not obtained.